Measuring impedance as a function of frequency is generally used for obtaining data on the operation of various electrical components. One example of such component is a Film Bulk Acoustic Wave Resonator (FBAR) device which is based on the Bulk Acoustic Wave (BAW) technology. FBARs are easy to implement as monolithic structures, for example, on CMOS circuits. High resonance frequencies and quality factors are achievable using the FBAR technology. FBAR devices can be used, for example, as sensitive mass sensors because the impedance of the resonator changes as matter is positioned on a mass-loading area of the sensor. If an (bio)active layer is deposited on the mass-loading area of FBAR, one may achieve selectivity of substances to be measured and thus a selective (bio)sensor.
Traditionally, the impedance of FBAR sensors and the like is measured using laboratory-scale equipment, such as impedance analyzers or circuit analyzers, which measure the impedance of the component using very definite frequency excitation.
One solution for measuring the impedance of a resonator-type components or components that can be connected as part of a resonator, is to use so-called oscillator couplings. In the case of mass sensors, the purpose of such couplings is to determine the series or parallel resonance frequency of the oscillator as a function of mass change of the component. However, in practice it is often difficult or impossible to implement an operational and accurate oscillator coupling in particular as an integrated, ie. on-chip, structure. This is mainly because of the following reasons:                Resonators typically have inherently low coefficient of coupling or quality factor (Q-factor). A particular problem is related to measuring liquid-form samples, as the presence of liquid on a resonator drastically reduces the quality factor of the resonator.        Some components, such as FBAR sensors typically have several parallel resonance frequencies.        Resonators may have relatively large manufacturing tolerances, resulting in that their series/parallel resonance frequencies vary.        Large parasitism and parallel capacitance cause that the change of impedance as a function of frequency is relatively small. In addition, the phase will in practice never shift 180 degrees.        
The above disadvantages apply in particular to FBAR sensors designed to be used as mass sensors, but may apply to other types of components too. Consequently, in practice oscillator couplings for impedance measurements can be taken advantage of only in the very limited case of high Q-factor resonators and absent or eliminated parallel resonances. In addition, very specific designs having low dynamic range must be used.
WO 2007/030462 discloses an interrogation circuit for inductive loads, comprising a voltage-controlled oscillator, a grid dip oscillator and phase locked loop. When signalled by the grid dip oscillator, the phase locked loop stops following the interrogation signal and remains producing a locked signal. The locked signal is passed to a frequency counter. Thus, by means of such circuit, measurements only at a point frequency can be carried out. In addition, the circuitry required by such a solution is relatively complex and not as such suitable for sensor devices having the interrogation circuit integrated therein as a single monolithic structure. The circuit is also expensive and as such not suitable to be used in connection with disposable sensor devices.